(1) Field of the Invention
This invention relates to a line-sequential color signal discrimination circuit which, when two kinds of color signals are alternately selected every other horizontal scanning period to constitute a single signal or a so-called line sequential color signal, discriminates as to which one of the two kinds of color signals is selected in each of the horizontal scanning periods. More particularly, the present invention relates to a discrimination circuit of the kind above described which, when applied to, for example, a recording and reproducing system, recording a color video signal on a recording medium in a line sequential fashion and then reproducing the recorded signal from the recording medium, is useful for discriminating as to whether the signal reproduced in each of the horizontal scanning periods is which one of the two kinds of color signals, for example, the color difference signals R-Y and B-Y.
(2) Description of the Prior Art
In countries including France, a line sequential television system called the SECAM system is a standard television system. According to this SECAM system, two color difference signals B-Y and R-Y are alternately switched over every other horizontal scanning period and are used for frequency modulation of two subcarriers having slightly different frequencies respectively, and a luminance signal is superposed on said frequency-modulated color signals to constitute a carrier video signal. In a demodulation system demodulating said carrier video signal, the color signal delayed by one horizontal scanning period and the color signal not subjected to the delay are derived in parallel relation, so that the color difference signals R-Y and B-Y dropped out at the time interval of one horizontal scanning period at the time of modulation are complemented by the color difference signals R-Y and B-Y demodulated in the preceding horizontal scanning periods to obtain the two independent and continuous color difference signals R-Y and B-Y. For this purpose, the demodulation system includes a switch for alternately deriving the color signal delayed by one horizontal scanning period and the color signal not subjected to the delay. Deriving the two kinds of color signals in parallel relation as described above is called the simultaneity, and the switch provided for that purpose is called a simultaneity switch.
The simultaneity switch used in the SECAM system is changed over in a manner as will be described presently. That is, the two color difference signals R-Y and B-Y alternately changed over every other horizontal scanning period to constitute the line sequential color signal are used for modulating subcarriers having respectively different frequencies, and, in the demodulation system, an identification signal discriminating between the color difference signals R-Y and B-Y on the basis of the frequency difference is produced to change over the simultaneity switch.
For the generation of such an identification signal on the basis of the frequency difference between the subcarriers, it has been a common practice that the burst signal of the color signal is passed in parallel relation through two narrow-bandpass filters having different passbands, and the outputs of these filters are compared to produce the identification signal. In this case, the passband of each of the bandpass filters must be as narrow as possible in order that the subcarriers can be accurately derived without being affected by noise and the like and in a relation distinctly separated from each other. However, narrowing of the passband of each of the filters results in a great delay of the filter output, and the problem arises in which the switching timing of the simultaneity switch is delayed, and one of the color difference signals is sequentially included in the other color difference signal. Such a problem occurs not only in the SECAM television system but also occurs naturally in a system including the steps of arranging two color difference signals in a line sequential fashion, frequency-modulating two subcarriers having a slight frequency difference by the two color difference signals respectively, combining these two FM signals into an FM line sequential color signal, recording this FM line sequential color signal together with an FM luminance signal, obtained by frequency modulation of a main carrier by a luminance signal, on a recording medium in a frequency division multiplex (FDM) mode, and then reproducing the recorded composite video signal.